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It's calling for 8" at the burner? Seems pretty high for that type of burner. And you adjusted that at max fire? If that is all good, have you tried lowering the airflow abit, just to see what would happen. With a VFD is would be real easy. Also what the flame look like at minimum and maximum fire? It should real steady, if not airflow or gas pressure isn't right(assuming the burner is good, which it does from the picture)

I can't see how upgrading to a pilot setup with solve your problem, but again I don't have much experience directly lighting those burners. So who knows, maybe it's a bad setup and that why the manufacture is suggesting a pilot upgrade.

when fails, always fails on ignition. can hear spark, but does not light. The burner orfices have been cleaned. Gas press. after honeywell valve is 8.5"wg.

Ok now we are getting somewhere.

Need to be 100% sure that the spark electrode gap is per factory.
Need to be 100% sure that spark is at electrode end and not sparking through wire to ground or at any other point of origin. (Per your pic it appears you have the spark electrode assembly with remote flame sense. make sure the electrode ground bar is not broke or missing or the tip burnt off I have attached the fenwal manual with all the good poop in it. If all is good?????

On ignition when your control is sparking with your volt meter on the low fire valve, make sure you have voltage (when failing) and you actually are moving gas through low fire valve. (You know where im heading) and it is reaching your burner orphices. Just confirming we dont have a failing low fire valve.

Now you have me just as curious to see this resolved without parts changing unless necessary.